Modular drainmast for aircraft

Abstract: A modular drainmast assembly for an aircraft includes a drain tube assembly (304) removable enclosed in a fairing (302) that is removably attachable to an aircraft. The electrically heated drain tube assembly (304) may be accessed in the field for service, repair or replacement. A resilient sealed interface (309) supports the drain tube where it exits the drainmast while permitting relative movement between the drain tube and the drainmast fairing (302) due to thermal expansion and contraction.

Description: This invention pertains to a drain apparatus, particularly a drainmast for aircraft, and particularly to a modular drainmast including a drain tube removably enclosed in a fairing that is removably attachable to an aircraft. A drainmast is used to eject waste water from an aircraft waste system during flight without impingement of the ejected fluid further aft on the body of the aircraft, and to drain the waste system when on the ground. As used herein the term "waste water" means undesired water-based liquids that accumulate during operation of an aircraft on the ground and in flight. Waste water is generated by many sources, one common source being the aircraft galley in which water, soft drinks, coffee, wine, orange juice and other potable liquids are collected. Waste water can also accumulate from engine and other air inlets that trap water from condensation or ingest rain. A drainmast generally comprises a drain tube enclosed within an aerodynamically shaped, drag minimizing fairing that when installed protrudes downwardly from the underside of an aircraft such that its outlet extends into the surrounding airflow stream during flight. It may be swept aftwardly of the aircraft with the direction of airflow. The drain tube is coupled to the aircraft waste water storage tank and is heated to avoid freezing. In certain applications waste water is accumulated until it reaches a certain volume. Thereupon a valve inside the aircraft opens and the waste water drains from the tank through the drainmast. In other applications, the waste water is continuously drained. There are two prevalent configurations for drainmasts. One is herein referred to as a bottom discharge mast. The other configuration is herein referred to as a rear discharge mast. In a bottom discharge drainmast, the waste water is ejected generally perpendicularly to surrounding airflow. The mast is generally straight and swept. The axis of the drain tube at the exit forms an acute angle with the axis at the entrance. The angle is usually close to the angle of sweep of the mast. In a rear discharge drainmast, the mast is swept and wing shaped and tapers into an exit tube at the tip that is roughly parallel to surrounding airflow. Water is ejected parallel to the surrounding airflow from the rear of the mast. Since the axis of the drain tube at the exit is nearly perpendicular to the axis at the entrance, the drain tube is either bent in a smooth curve or is fabricated from two or more straight sections that together form the appropriate angle. The rear discharge mast is believed to be preferable to the bottom discharge mast. A portion of the water ejected from a bottom discharge mast has a tendency to adhere to the tip of the mast and dribble back driven by surrounding airflow. Much of it can freeze to the mast before it is blown away. In contrast, a properly designed rear discharge mast generates a suction action as surrounding airflow passes around the exit tube. Ejected water is sucked out of the tube into surrounding airflow. Very little if any water adheres to the mast. Drainmasts have been used on aircraft for many years. A common design developed for early commercial jet aircraft and still in use today is a rear discharge configuration and includes a fairing consisting of two shaped aluminum halves that are fastened together with screws. Each fairing half has a channel that receives the drain tube. The drain tube is formed from stainless steel or copper. Silicone elastomer tape is wrapped around the tube in several locations. The tape compresses when the halves are screwed together and firmly locates the tube in the mast. Because clearance between the tube and the mast is very small, considerable heat loss to and through the fairing occurs. The drain tube is either bent or fabricated from multiple sections. The drain tube cross section is oddly shaped ranging from oblong to triangular. Some designs have a cross section that changes between the inlet and the outlet. This design is suspected of being more susceptible to clogging than drain tubes of constant cross section. The drain tube is locally heated by fixed resistance ceramic type resistors. Heating along the length of the drain tube is provided only by heat conduction along the tube. A heated portion of the drain tube extends out of the drainmast at the tube exit to prevent freezing of residual water. Sustained temperature of the drain tube can reach 371 °C (700 °F). The exterior of the drainmast can reach temperatures on the ground which present a hazard to service personnel. Some designs are open at the top where the drainmast mates with the aircraft. This can present a fire hazard if hydraulic fluid or lubricating oils leak into the drainmast and contact the heating elements. Another known drainmast design is of bottom discharge configuration and has a fairing that is formed from two pieces of compression molded fiber reinforced plastic. The two halves are fastened together with adhesive and screws. The drain tube is copper, has a constant round cross section, and is attached to a shaped metal tip that covers the end of the mast. The heating element is wire or foil encapsulated in silicone rubber. The heating element is bonded to the outside of the tube and inside the metal foot that covers the end of the drainmast. The heated metal foot is required to prevent discharged waste water residue from freezing to the drainmast. As discussed previously, this is an inherent problem with bottom discharge drainmasts. The drain tube is sandwiched between the fairing halves which are mated to the metal foot. The entire assembly is filled with expanding foam. The resulting unit is integral and cannot be disassembled without damaging the components. The top of the unit is sealed where it mates with the aircraft. A controller inside the aircraft cycles power to the heating element to maintain temperature within preset limits. Power is transferred to the drainmast via a connector that is attached to the mounting base of the mast. A more recent known drainmast design also utilizes a fiber reinforced plastic fairing. Mirror image fairing halves are formed from resin impregnated fabric that is laid into a female mold, vacuum bagged, and cured under heat and pressure, and trimmed after molding. The drain tube is formed from fiber reinforced plastic and has an integral heating element encapsulated in fiber reinforced plastic and bonded to the outside of the tube. The fairing halves are bonded together with the drain tube sandwiched in between. Lightweight filler material such as polyurethane expanding foam occupies the void in between the tube and fairing. The resulting drainmast is integral and cannot be disassembled without damage or destruction of the drainmast assembly. A heated foot is not required because the rear discharge feature is utilized. The heating element for this drainmast design is a pattern of wire that has a high positive coefficient of electrical resistivity. As the element heats the resistance increases which causes supply current to decrease. A steady power consumption and drain tube temperature is reached the values of which depend on the operating environment of the drainmast. This feature is referred to as a self regulating heating element. An electrical controller inside the aircraft is not required which greatly simplifies the system. A thermoswitch may be located inside the drainmast. The thermoswitch is operably connected to cause opening of the power supply circuit when the thermoswitch temperature is above a preset value to prevent current from flowing to the heating element when outside air temperature is above freezing. This feature is desirable to prevent overheating the fiber reinforced plastic fairing. It also serves to decrease overall power consumption of the drainmast. All electrical components are encapsulated inside the drainmast and cannot be removed. Yet another recent known drainmast design has a fairing consisting of fiber reinforced plastic that is resin transfer molded in one piece. As in other known embodiments the drain tube and thermoswitch are integrally molded in place inside the fairing during the resin transfer molding process and cannot be removed without damage or destruction. As described hereinbefore, in certain known constructions a drain tube assembly is molded into a fairing as a part of the process of resin transfer molding of the fairing. Cyclic loading from thermal expansion and contraction of the drain tube during heating and cooling is transferred directly to the fairing and cracking of the fairing has been known to occur. A drainmast avoiding the cracking of the fairing is disclosed in EP-A-0 556 765 on which the precharacterizing part of claim 1 is based. This drainmast comprises a hollow fairing in which a drain tube assembly is disposed. The drain tube assembly includes a drain tube and an electrothermal heating element. The drain tube is supportively affixed to the fairing proximal to a mounting base of the fairing and extends through the hollow fairing without touching the fairing walls. An elastomeric fitting is located in an exit hole of the fairing which receives and resiliently supports the end of the heated drain tube within the fairing at the exit hole. However, the fixture of the drain tube proximal to the mounting base requires exacting tolerances causing high production costs. It is the object of the present invention to provide a drainmast and a drain tube apparatus having an increased durability and safety which can be manufactured with low production costs. This object is solved, according to the invention, with the features of claim 1. The present invention allows at least one end of the drain tube to move relative to the fairing, freely expanding and contracting without resistance, allowing greater tolerances. Simultaneously the drain tube is guided so that it may not touch the fairing walls. This solution is not restricted to a modular construction and could be applied equally as well if the drain tube were permanently attached to the fairing, for example by bonding the attachment plate to the mounting base of the fairing. Preferrably, the drain tube is free to move at both ends relative to the fairing. Durability and safety are further increased by the construction of the sealing of the drain tube assembly in the drainmast. By preventing liquids from entering the drainmast, corrosion and fire hazards are avoided. The features and advantages of the present invention will become more apparent when viewed in the light of the description of the embodiments of the invention in connection with the drawings which form a part of the specification. In the drawings: Figure 1 is a side view of a drainmast assembly according to the invention. Figure 2 is a top elevation of Figure 1, Figure 3 is an enlarged cross section taken on line 3-3 of Figure 1, Figure 4 is a right end elevation of Figure 15, Figure 5 is an enlarged fragmentary cross section taken along line 5-5 of Figure 2, Figure 6 is a side view of the drain tube of the Figure 1 drainmast assembly, Figure 7 is a schematic diagram of an electrical circuit for the embodiment of the invention of Figure 1. In the drawings, the rear of the drainmast is defined as that portion furthest downstream relative to surrounding airflow when the drainmast is installed on its intended airplane that is in flight. In the drawings like reference numerals designate like or corresponding parts throughout the several views. A rear discharge drainmast assembly 300 according to an embodiment of the invention is presented in Figure 1. Drainmast 300 comprises fairing 302 and drain tube assembly 304. Fairing 302 includes a mounting base 303, a mast 305, and terminates in an exit tube 301. Mounting base 303 includes a plurality of mounting holes 307 for attaching the drain mast to an aircraft. In this embodiment, the fairing 302 is formed in two pieces: a fairing body 302' and a cover plate 302''. The cover plate 302'' separates from the fairing body 302' along parting line 320. The two pieces together define a hollow cavity 328 inside the fairing. They are joined together by screws 324 and mounting posts 322. Attachment means, which are also referred to as spacer means or clamps 326, serve to join the two pieces, to fix their distance and to guide the drain tube assembly 304 within the fairing 302. The drain tube assembly comprises a drain tube 306 and a foot 308 which include electrical heaters. The drain tube assembly 304 exits both ends of the fairing 302. An adapter fitting 310 serves to adapt the drain tube assembly 304 to the aircraft waste water system. In this embodiment, adapter fitting 310 is formed from metal and is rigidly attached to the drain tube 306. A first resilient sealed interface 309 is located between the foot 308 and the fairing 305. This interface serves the same purpose as described in relation to elastomeric fitting 12 of Figure 1 of EP 0 556 765. It permits relative thermal expansion and contraction between the drain tube assembly 304 and the fairing 302 while maintaining a seal and supporting the drain tube in the mast exit hole. At the opposite end, a second resilient sealed interface 332 is located between the fairing 302 and the drain tube assembly 304. Resilient sealed interface 332 supports the foot 308 (and drain tube assembly 304) relative to mounting base 303, enables relative movement between the drain tube assembly 304 and the fairing 302 due to thermal expansion and contraction, seals the fairing 302 against entry of fluid exiting the tube, and facilitates removal and reinstallation of the drain tube assembly 304 for service or replacement. In such manner, both ends of the drain tube assembly 304 are supported, but are free to "float" relative to the fairing while maintaining a seal against liquid penetration into the cavity 328. A power line 313 enters the drain mast 300 through sealed hole 318 in mounting base 303 and terminates in connector 315 outside the fairing 302. Connector 315 mates to the aircraft electrical and control system. A ground line 316 is electrically connected to the drain tube assembly 304 and exits the fairing through sealed hole 319. The drainmast assembly 300 is thus sealed against liquid entry through the top of the mounting base 303 into cavity 328. Figure 2 presents a view of the top of the drainmast. Mast 305 extends below mounting base 303. Cover plate 302'' joins to fairing body 302' along parting line 320. In the embodiment presented, second resilient sealed interface 332 comprises an elastomeric fitting. The fitting is captive between cover plate 302'' and fairing body 302'. The elastomeric fitting includes an annulus with enlarged diameters on either end as shown in Figures 15 and 20. The mounting base 303 has mating surfaces that trap the fitting in place when the cover plate 302'' is joined to the fairing body 302'. Figure 3 presents a cross sectional view of one of the attachment means 326. Cover plate 302'' joins fairing body 302' along parting line 320 forming cavity 328. An electrical heater 330 is wrapped around the circumference of drain tube 305. A metal element, preferably wire, is imbedded in an electrical insulation layer, preferably silicone elastomer. An outer insulation layer is optional. A resilient layer 334 may be formed around the drain tube assembly 304 at the spacer locations. The resilient layer 334 serves to locate and resiliently support the drain tube assembly 304 when clamped between the cover plate 302'' and fairing body 302'. Resilient layer 334 is most preferably formed by wrapping silicone elastomer tape around the drain tube assembly at the clamp locations allowing the drain tube assembly to move relative to the fairing while freely expanding and contracting without resistance during heating and cooling of the drain tube assembly. A preferred silicone elastomer tape is catalog number 2650 available from Permacell, a Nitto Denko Company, New Brunswick, New Jersey. The attachment means 326 is split along parting line 336 with half being integrally formed with the cover plate 302'' and half being integrally formed with the fairing body 302'. Screws 324 draw cover plate 302'' down thereby establishing the clamp action. Such clamping arrangements may be varied without effecting the practice of the invention as long as the drain tube assembly 304 is permitted to float relative the fairing 302. Figure 4 presents a rear view of the drainmast 300 attached to and extending beneath the aircraft structure. Mast 305 protrudes from mounting base 303 into a surrounding airflow. Adapter fitting 310 attaches to a flexible hose (not shown) that is part of the aircraft waste water system. Ground line 316 is electrically connected to the aircraft structure. Figure 5 presents a detailed cross section of the drainmast 300 where the drain tube assembly 304 exits the fairing mast 305 along line 5-5 of Figure 2. Drain tube 306 terminates with a foot flange 342. Foot ring 344 extends circumferentially around foot flange 342. In one preferred embodiment, the drain tube 305 is formed from stainless steel and the foot ring 344 is formed from copper. The two are welded together along the seam where they meet each other. Other metals may be used to form the drain tube 306 and foot ring 344 which may permit other fabrication techniques such as brazing or soldering or integral formation by spinning or deep drawing. They might also be formed from fiber reinforced plastic in which case the foot ring 344 could be integrally molded with drain tube 306. A foot heater 340 is circumferentially located inside the foot between the drain tube 305 and foot ring 344. Foot heater 340 includes a foot heating element 341 encapsulated in electrical insulation, preferably silicone elastomer. For ease of manufacture, the drain tube heater 330 is separately formed and includes a drain tube element 333 encapsulated in electrical insulation, preferably silicone elastomer. The drain tube heater 330 and foot heater 340 circumferentially abut at 360. A lip 346 extends from end of the exit tube 301 similar to lip 234 of Figure 13. The first resilient sealed interface 309 is located between lip 346 and foot ring 344. A hardened liquid sealant is preferred for interface 309 which permits relatively larger misalignment between foot 308 and mast 305 during manufacturing. In particular, foot ring 344 may shift laterally out of alignment with mast 305 while maintaining the required seal. This feature greatly facilitates the manufacturing process. Figure 6 presents drain tube assembly 304. Drain tube heater 330 is wrapped around the circumference cf the drain tube 306 along the length of the tube. Foot heater 340 is similarly wrapped around the drain tube 306 and inserted into the foot 308 as discussed above in relation to Figure 5. Drain tube heater 330 and foot heater 340 circumferentially abut at interface 360. The power and control line 313 is shown severed for the sake of clarity. Drain tube heater 332 and foot heater 340 are electrically connected in series, described as follows. Power line 314 is electrically connected to thermostat 350. Line 352 electrically connects the thermostat to the foot heater 340 at 354. The return line (not shown) from the foot heater 340 is electrically connected to the drain tube heater 332 at 354. Power line 312 is electrically connected to the drain tube heater 332 and completes the series circuit. As before, the preferred heater element material for the drain tube heater 332 and foot heater 340 is heater wire having a composition of 70% nickel and 30% iron. A fully self regulating heater may not be possible for all applications, and a thermostat may be required. In such a case, the wire specified above helps to control thermal transients and creates at least a partially self regulating drain tube heater 332 and foot heater 340. In a certain preferred embodiment, the thermostat is set to open the heater circuit at about 67 °C (153 °F) and to close the heater circuit at about 56 °C (133 °F). A ground line 316 (shown severed for ease of illustration) is electrically connected to a ground bracket 317 by suitable means. Ground bracket 317 is attached to drain tube 306 by suitable means, such as welding, brazing, or soldering. A liquid sealant may be provided between elastomeric fitting 332 (the second resilient sealed interface) and the drain tube 306. In some applications, it may be desirable to use a liquid sealant, such as PR-1422 polysulfide sealant or Dow Corning 738 RTV silicone elastomer, to form the second resilient sealed interface 332. Referring to Figure 1, the fairing 302 may be formed from metal or plastic, but fiber reinforced plastic is particularly preferred. Fairing 302 may be formed by compression molding, resin transfer molding, or other suitable molding techniques. In a certain preferred embodiment, the cover plate 302'' and fairing body 302' are separately compression molded using a chopped fiberglass and/or graphite fiber in polyester or epoxy resin molding compound. The drainmast assembly 300 is fabricated by applying a liquid sealant bead, preferably a polysulfide, around the fairing lip 346 (Figure 5) and around the circumference of elastomeric fitting 332 between the two enlarged outside diameters. The drain tube assembly 304 is then placed in the half of clamp 326 (Figure 3) located in the fairing body 302'. A bead of polysulfide sealant is then applied around the circumference of fairing parting line 320 and to all screw locations. The cover plate 302'' is then placed over the fairing body 302' and drawn down by tightening screws 324. The fairing is thereby completely sealed, except for a breather hole 329 that vents cavity 328 to the atmosphere. A schematic circuit for the drainmast 300 is presented in Figure 7. Electrical power at 115 VAC is supplied to power line 314 and transferred to thermostat 350. Thermostat 350 controls the power to foot heater 340 and, drain tube heater 330 which are grounded to the aircraft electrical system via power line 312. Drainmast 300 is grounded to the aircraft via ground line 316.

Claims: A drainmast for discharging fluid into a surrounding airflow comprising: a fairing (302) having first and second opposing ends, a drain tube assembly (304) disposed within said fairing (302) and exiting said fairing (302) at said first and second ends, said drain tube assembly (304) including an electrothermal heating element (330), a drain tube attachment (332) that attaches said drain tube assembly (304) to said fairing (302), allowing at least one end of the drain tube to move relative to the fairing, freely expanding and contracting without resistance, and a first resilient sealed interface between said drain tube assembly (304) and said first end, characterized in that said fairing (302) comprises a fairing body (302') having an opening and a cover plate (302'') covering said opening, said fairing (302) enclosing attachment means (326) providing openings for resiliently guiding said drain tube assembly (304) within said fairing (302). The drainmast of claim 1 wherein the distance between the fairing body (302') and the cover plate (302'') is fixed by the attachment means (326) disposed within said fairing (302). The drainmast of claim 1 or 2 wherein a resilient layer (334) is located within the opening of said attachment means (326) surrounding the drain tube assembly (304). The drainmast of one of claims 1-3 wherein the attachment means (326) are comprised of an upper and a lower half, the upper half being integral with the fairing body (302') and the lower half being integral with the cover plate (302''). The drainmast of claim 4 wherein the upper and the lower half of the attachment means are maintained in relative abutment by screws (324) extending from outer surface of the fairing into the opposing spacer half. The drainmast of one of claims 1-5 wherein the drain tube assembly (304) comprises a foot (308) with a foot ring (344), the foot ring (344) surrounding the end portion of the outer circumference of the fairing (302), with a resilient sealed interface (309) disposed between the fairing (302) and the foot ring (344). The drainmast of claim 6 wherein the end portion of the fairing (302) is recessed so that the outer surface of the fairing (302) is flush with the outer surface of the foot ring (344). The drainmast of one of claims 1-7 further comprising a second resilient sealed interface between said drain tube assembly (304) and said second end, the drain tube assembly (304) and the fairing (302) being sealed at both ends of the fairing (302). The drainmast of one of claims 1-8 wherein said first resilient sealed interface (309) is a hardened liquid sealant. The drainmast of claim 9 wherein said hardened liquid sealant is chosen from the group consisting of polysulfide sealant and room temperature vulcanizing silicone elastomer sealant. The drainmast of one of claims 1-8 wherein said first resilient sealed interface (309) is an elastomeric fitting. The drainmast of one of claims 1-11 wherein said drain tube attachment resiliently attaches said drain tube assembly (304) to said fairing (302). The drainmast of one of claims 1-12 wherein said drain tube assembly (304) includes a foot (308) exposed to the surrounding airflow, said foot (308;208) including an electrothermal heating element. The drainmast of one of claims 1-13 wherein said fairing (302) is a plastic, preferably a fiber reinforced plastic fairing (302).

Inventors: GIAMATI MICHAEL JOHN, GIAMATI, MICHAEL JOHN
Applicants: GOODRICH CO B F, THE B.F. GOODRICH COMPANY
Application year: 1994
Main classification: B64C1
IPC codes: B64D11, B64C1, B64D1